Nowadays the term “composite materials” or “composites” is often heard when referring to fibre-reinforced polymer materials. Originally, however, the term refers to materials made from two or more constituent materials with significantly different chemical or physical properties, that, when combined, produce a material with different characteristics from the individual components. Although there are also metal composites, the term “composite materials” will hereafter in this document mainly refer to materials that are (partially) transparent for “terahertz radiation”. These materials include glass fibre and ceramic materials, as well as different kinds of materials with internal “honeycomb structure” and foams, which are widely used nowadays in especially the aerospace, automotive, nautical and wind energy industries, as well as in building construction and for the production of electronic components. The use of these materials is ever increasing because of their advantageous characteristics like low weight, high strength and high durability. Many essential parts of airplanes, cars, ships, spacecraft and wind turbines are presently made of (assemblies of) composite materials. Safety regulations require that such assemblies of composite materials are regularly inspected for timely detection of possible internal or external structural flaws. Conformity testing even requires that every product undergoes such inspection when it leaves the production line. In most cases it is undesirable to temporarily remove a composite assembly from the structure of which it forms a part, for inspection, for instance because of the operation schedule of an aircraft, or because of the size of the composite assembly, for instance in case of a ship hull. A solution to this is to bring the inspection equipment to the composite assembly that is to be inspected. This is called “non-destructive testing”. In case the inspection method does not require any contact between the inspection system and the composite assembly, this is called “non-contact testing”. However, the method according to the present invention is also suited for contact testing. In some situations it may be important that changes in material conditions and structural integrity are detected as they occur and can be monitored in real time. This is called “condition monitoring”. The method according to the present invention can also be advantageously applied in situations where such condition monitoring is required.
Some of the more traditional non-destructive inspection methods, known from prior art, are the ones using for instance X-rays or ultrasound. The main drawback of the use of X-ray inspection of composite assemblies is the need for additional safety requirements which makes large scale application less feasible commercially. A serious drawback of the use of ultrasound for inspection of composite assemblies is the fact that the ultrasound waves are scattered (no transmission, no reflection), partially or completely, by many of the adhesives that are frequently used in composite assemblies. Because of the mentioned drawbacks, X-ray and ultrasound inspection are not suitable for many applications where inspection of composite assemblies is required.
At the end of the 1990's so-called “terahertz imaging” emerged as a tool for inspection of material structures. The THz frequency range is the frequency range between, approximately, 10 GHz and 10 THz, between the microwave and infrared parts of the spectrum. The use of THz radiation makes excellent contrast mechanisms and high resolution imaging possible. Non-polar liquids, dielectric solids and gases are at least partially transparent for THz radiation, while metallic surfaces totally reflect THz waves. The main fields of application were originally security (detection of the material of, for instance, concealed weapons and explosives) at airports, analysing biological materials, analysing dielectric materials, determining geometrical properties of objects, like length, width and thickness, determining material properties like density and contamination and imaging of objects and material samples (for instance layered structures).
Initially the equipment to be used to produce, detect or process terahertz radiation was very expensive, hence widespread use was not economically feasible and remained limited. Since about 2005 this gradually changed because of new technological developments in the THz field, and an increasing number of THz systems and components have been developed and patented since then.
The object of the present invention is to present a method for non-destructive, contact or non-contact inspection of composite assemblies using terahertz radiation, whereby at least one of the materials in the assembly is (partially) transparent for said terahertz radiation, which method is independent of the way that the composite materials forming said composite assemblies were joined together, and without the need for a priori knowledge about the structure, shape or configuration of said composite materials (for instance layered, foam, placed on metal substrate etc.)
From prior art several methods are known for inspection of material structures using THz radiation. For instance the document US 2007/0090294 A1 describes a method where a liquid is applied to the surface of an object under test, whereby the said liquid absorbs THz radiation in a different manner than the structure/material of the object under test and thus provides the contrast for THz imaging. A significant drawback of the said method is the fact that it is limited to inspection and detection of surface material conditions, like cracks and recesses, that are in direct contact with the object surface so that the said liquid can penetrate. Another serious drawback is the fact that the method requires that excess liquid is removed from the object before imaging. This requires cumbersome actions like wiping, heating or blow-drying. The method according to the present invention is not limited to surface inspection and requires no additional actions like the use of fluids for imaging contrast purposes.
Another prior art document, U.S. Pat. No. 7,876,423 B1, is directed at detecting microstructural and thickness variations in dielectric materials using THz energy. The described method is mainly directed to the inspection of sprayed-on foam on a metal container, i.e. the Space Shuttle external fuel tank thermal protection and is not specifically directed at the inspection of composite assemblies. The method according to the present invention does not require prior knowledge about the structure, shape and configuration of the materials under inspection. It suffices to know that at least one of the materials in the assembly under inspection is (partially) transparent for terahertz radiation.
The non-patent prior art references C. Jansen, S. Wietzke, H. Wang, M. Koch and G. Zhao, “Terahertz Spectroscopy on Adhesive Bonds”, Polymer Testing, 2011, 30(1), pp. 150-154, and S. F. Durrschmidt, S. Wietzke, C. Jansen, H. Wang and G. Zhao, “Terahertz testing of adhesive bonds”, 36th. International Conference on Infrared, Millimeter and Terahertz waves (IRMMW-THz), 2011, pp. 1-7, are concerned with inspecting the integrity of adhesive bonds between polymer materials. It should be clear to the reader that the method according to the present invention is not limited to the inspection of adhesive bonds but is aimed at the detection and analysis of a wide range of material conditions within composite assemblies (for instance cracks, recesses, internal cavities, damaged adhesive joints, density or thickness variations, porosity, delamination, inclusions, etc.)
As such the method according to the present invention aims to provide solutions for the limitations and drawbacks associated with the prior art in the relevant field.
The present invention further relates to a system for non-destructive, contact or non-contact inspection of composite assemblies using terahertz radiation, said system according to the present invention being characterised in that the system comprises such electronic and electro-optical components that it can illuminate an assembly of composite materials with radiation having a frequency in the so-called “Terahertz range” (10 GHz-10 THz.), detect the radiation reflected from and/or transmitted through the said assembly of composite materials, record the spatially resolved reflection and/or transmission responses, construct a two or three-dimensional image from the said spatially resolved reflection and/or transmission components and present said image to the user of the system. This enables measurement of the amplitude and phase of the detected signal with very high dynamic range (>60-70 dB) and construction of high-detail output images.
The document A. M. Baryshev, W. Jellema, R. Hesper, W. Wild, “Reflection Measurement of Absorption Coatings using 600-670 GHz Vector Network Analyzer”, 18th International Symposium on Space Terahertz Technology, March 2007, California Institute of Technology, Pasadena, Calif., USA, discloses the construction and function of a Vector Network Analyzer (VNA) that is equipped with such tunable solid state submm signal sources, that the function of the VNA is extended into the terahertz frequency range. Some of the advantageous characteristics of the said extended VNA, that was developed by SRON—The Netherlands Institute for Space Research, are employed within the system according to the present invention, to enable the forming of a two or three-dimensional image of the material structure of an assembly of composite materials, from which image detection and analysis of material conditions of the composite materials forming said composite assemblies is possible, provided that at least one of the composite materials in the assembly is (partially) transparent for said terahertz radiation, but irrespective of the way that the composite materials forming said composite assemblies were joined together, and without the need for prior knowledge about the structure, shape or configuration of said composite materials (for instance layered, foam, placed on metal substrate etc.). As such the method according to the present invention aims to provide solutions for the limitations and drawbacks associated with the prior art in the relevant field.